Updated: 25 Dec 2025, 05:21 UTC

AFDPRT
EXPERIMENTAL AFD FOR PORTO
EXPERIMENTAL FORECAST OFFICE - PORTO METRO
05:21 UTC THU DEC 25 2025

Area Forecast Discussion - Porto

Issued: 05:21 UTC | Valid through: 05:21 UTC MON JAN 6 2026

SYNOPSIS

A shallow mid-level ridge extends over the eastern North Atlantic through the end of December, maintaining broadly stable conditions across coastal northern Portugal. A weak shortwave trough is expected to approach from the northwest late on 2026-01-01, initiating a moistening trend aloft. This will be followed by a deeper, progressive upper-level trough rotating through the central Atlantic during the 03–06 JAN period, increasing cyclonic vorticity advection over Iberia. Surface pressures remain near seasonal normals, with weak baroclinic zones forming in the marine boundary layer ahead of approaching frontal systems. Low-level moisture return increases gradually from the southwest, supported by a strengthening low-level jet ahead of the 06 JAN cyclone. Despite modest large-scale ascent, model soundings indicate limited instability and weak lapse rates, favoring stratiform over convective precipitation. Near-surface temperatures remain near or just above freezing during coldest nights, with wet-bulb temperatures frequently dipping to 1–3°C, raising potential for brief frost or freezing drizzle on elevated surfaces under calm, clear conditions.

NEAR TERM (0–24 hours)

As of 05:21 UTC, the immediate environment over Porto is characterized by clearing skies following early morning cloudiness, with surface temperatures around 9°C and gusty winds near 40 km/h. The current cloud deck, which peaked around 04–05 UTC, is eroding rapidly under subsidence associated with the western flank of the Atlantic ridge. Insolation will be strong during the late morning and early afternoon due to diminishing cloud cover, supporting diurnal heating to a high near 11°C. Boundary layer mixing is expected to deepen, reducing low-level humidity and limiting any residual shower potential.

Precipitation chances remain at 15% through midday, primarily tied to weak, transient mesoscale convergence zones along the coast—features poorly resolved in convection-allowing models and subject to high spatial variability. Any showers would be brief and isolated, with negligible accumulation.

The primary thermal concern is the overnight period into 26 DEC. Clearing skies, light winds after 00 UTC, and dry surface conditions will support efficient longwave radiation loss. Minimum temperatures are expected to fall to 3°C, with wet-bulb values near 1°C. Frost cannot be ruled out on grassy or elevated surfaces, particularly in rural and suburban microclimates where cold air drainage is unimpeded. Pavement temperatures are expected to remain just above freezing due to urban heat retention.

Model guidance has stabilized over the past 3 hours, with recent runs (05:05–05:16 UTC) converging on a dry, clear day with minimal temperature spread (low: 3–4°C, high: 11°C). Confidence in the near-term evolution is high, though exact timing of cloud dissipation and wind reduction remains sensitive to boundary layer mixing efficiency in the WRF and AROME-EPS solutions.

SHORT TERM (24–72 hours)

From 26 DEC through 28 DEC, the ridge axis shifts westward, allowing a series of weak shortwaves to pass north of Iberia. These will induce modest diffluent flow aloft but lack significant amplitude or thermal contrast to generate widespread ascent. The 12–18 UTC period on 26 DEC shows a slight increase in low-level moisture advection, with precipitable water values rising from 12 to 15 mm. This coincides with the peak in probability of precipitation (38%) during the late morning and early afternoon hours. However, model soundings (ECMWF HRES, GFS 0.25°) show a capping inversion near 850 hPa and weak mid-level lapse rates, suppressing deep convection. Any precipitation will be light, brief, and likely virga or isolated drizzle, especially near the coast where marine stratocumulus may briefly lower.

Winds increase on 27 DEC due to tightening pressure gradients between a weak surface low over the Bay of Biscay and the Azores high, with gusts to 48 km/h possible in exposed coastal zones. These winds may limit nocturnal cooling slightly, supporting a low of 5°C despite clear skies.

By 28 DEC, a weak warm advection pulse aloft increases boundary layer temperatures to 14°C, the warmest in the forecast period. Low-level moisture remains limited, and no precipitation is expected. The warm-up is transient, as the ridge weakens late on 29 DEC ahead of the next synoptic-scale impulse.

Ensemble spread (ECMWF EPS, GEFS) remains low for temperature and cloud cover through 28 DEC, but precipitation probabilities show moderate dispersion—some members develop a weak coastal front that stalls offshore, increasing drizzle potential, while others maintain dry conditions. The most likely outcome remains dry, but brief, localized moisture pulses cannot be ruled out.

LONG TERM (beyond 72 hours)

The pattern shifts notably beginning 01 JAN as a deep upper-level trough progresses eastward from the central Atlantic. This system is embedded within a broader amplifying wave pattern, with a downstream ridge over eastern Europe enhancing diffluence over the Iberian Peninsula. The approach of this trough increases mid-level cyclonic vorticity advection starting 01 JAN, with 500 hPa vorticity maxima passing 300–400 km north of Porto.

Low-level moisture return strengthens on 02–03 JAN as a southerly low-level jet develops ahead of the surface cyclone, which is forecast to track north of the Iberian coast. This supports prolonged periods of moderate drizzle, particularly on 02 and 05 JAN, with model QPF totals ranging from 7–12 mm. The 06 JAN event is the most robust, with GFS and ECMWF in agreement on a surface low passing near 42°N 12°W, driving a warm front toward northern Portugal. Precipitation is expected to be widespread and persistent, with 22.8 mm of QPF over 24 hours and gusts to 84 km/h. However, warm-air advection is modest—850 hPa temperatures remain near 6°C—so precipitation type remains liquid at all levels.

A brief cooldown follows on 04 JAN, with lows near 1°C and wet-bulb temperatures at 0°C. Clearing follows the passage of a weak cold front, but boundary layer moisture may linger, increasing frost potential on untreated surfaces. The 08 JAN period shows return to drier conditions as the upper pattern re-ridges.

Ensemble guidance shows increasing spread from 03 JAN onward, particularly in the track and intensity of the 06 JAN cyclone. The ECMWF ensemble leans slightly stronger and closer to the coast, while the GEFS is weaker and farther offshore. This introduces uncertainty in rainfall totals and wind speeds, though all members agree on a wet and windy period from 05–07 JAN.

CONFIDENCE & KEY UNCERTAINTIES

Near Term (0–24h): High confidence in dry conditions and diurnal temperature evolution. Frost potential on elevated surfaces is credible but spatially variable; confidence in pavement-level freezing is low.
Short Term (24–72h): Moderate to high confidence in overall pattern. Precipitation timing and coverage remain low-confidence due to marginal instability and small-scale forcing.
Long Term (72–96h+): Low to moderate confidence. Synoptic evolution is consistent across models, but exact phasing, coastal proximity, and intensity of the 06 JAN system remain uncertain. Precipitation totals could vary by ±30% depending on low track.
Winter Weather: No snow or ice is expected at sea level. However, on 04 JAN, with a low of 1°C and wet-bulb at 0°C under clear, calm conditions, brief freezing drizzle or frost on grass and elevated surfaces (e.g., bridges) cannot be ruled out. Accumulations would be negligible and non-impacting.
Model Oscillation: Recent model runs (last 30 minutes) show stabilization in temperature and QPF, with no significant oscillation. Earlier cycles had brief spikes in precipitation probability on 25 DEC, now resolved.

The primary forecast challenge remains distinguishing between dry, radiative cooling nights and weak moisture pulses that could suppress frost via cloud cover or light precipitation. Users should monitor boundary layer dew point trends and low-level wind profiles for real-time assessment of frost potential.

Experimental discussion - not official guidance